Vertices collapse on themselves
It happens because one or several bones are influencing this part of your mesh when they should not.
Select the armature in Pose mode, shift select the object, switch to Weight Paint mode, select the bone called Chest, you'll see that its vertex group contains vertices that are part of the arm:

So in the N panel, choose Blend &gt; Subtract in order to remove these vertices from this group and it will work fine:

You could also shelect these vertices in Edit mode, go in the Properties panel &gt; Object Data &gt; Vertex Groups list, in the dropdown menu on the right of the list select Remove From All Groups, then assign to the group called Arm_L only.